Fixed scanf string buffer overflow possibility.
Prevented buffer overflow using scanf.
Added limit to string length which can be scanf-ed.
Changed the signature of insert().
Refactoring lab3.

1. Killed some TODOs;
2. Added fflush_stdin() helper function and corrected the way stdin buffer is
cleaned;
3. Added some assertions;
4. Checked scanf return value;
5. Changed the insert function signature;
6. Changed strcpy to strncat usage.